Kay,
Further to Colin's comments. I presume you have BW's birth certificate as you have his dob. If you don't then search on FreeBMD.rootsweb.com and you will find his GRO entry: March 1868 page 2c vol. 40 Southampton. Further more if you just search for Farghers in Southampton between the years 1867 and 1901 you will find half a dozen more. They could be related to John and that could help narrow your search in the IoM.
You could also search for the family at the address on BW's birth cert in the 1871 Southampton census. I presume you have already looked for the family on the 1881 census.